In order to achieve an organic electroluminescence (EL) element of long life, it is known that one of the effective methods is to make a multi-unit type organic electroluminescent element (for example, refer to Non-patent document 1).
As applications of this organic electroluminescent element, there are many kinds of applications for outdoors as a single color light such as: an electric bulletin board, a traffic light, various lightings, a light for passenger car, motorbike, or bicycle, and a lighting source. From the viewpoint of efficiency, life, and reliability, LEDs, incandescent lamps, and fluorescent lamps have been used until now.
As a factor of decreased efficiency of an organic EL element and decreased light emission life with the same luminescence, the following are known. One is an absorption loss in which the emission light from the light emitting layer located near to the reflective electrode side is absorbed by the layers (in particular, organic layers) existing nearer to the transparent electrode side than the light emitting layer that emits the light. The other is a light degradation of the materials produced when the emission light is absorbed by the materials such as emission dopants contained in the light emitting layer located nearer to the transparent electrode side than the light emitting layer that emits the light.
Particularly, in a multi-unit type organic electroluminescent element having an emitting light of the same emission color, the emitting light from the reflective electrode side is absorbed by the emission dopant contained in a light emitting layer located in a light emitting layer nearer to the transparent electrode than the light emitting layer that emits the light. Further, by the light degradation of the dopant contained in the light emitting layer located in the transparent electrode side, decreased efficiency and decreased light emission life were produced.